An output driver is used to connect a digital signal to a load and to provide enough current for the digital signal to drive the load. The digital signal may be provided as the output signal of a digital device such as a memory device, a logic circuit, a microprocessor, or a gate array. In a common prior art static random access memory (SRAM) the output is connected to a pair of complementary metal oxide (CMOS) transistors. The CMOS drivers are, in effect, current amplifiers for the low current digital outputs of a memory device. The CMOS drivers boost the current of the digital output of the memory device from typically a few milliamps to enough, typically many 10s of milliamps, to drive the desired load, such as another device or an input to a display. Current gains of 100 (during switching) are common in these applications.
In the past designers of driver circuits have been able to neglect the effect of parasitic inductance. This inductance is generated by the wires and other current path connections disposed between the digital circuit and the externally applied signals (including power supplies), and between the driver and the load. Such neglect is acceptable so long as the current switched by the driver at the load is low and the propagation delay through the device is much greater than any parasitic LC time constant associated with drivers and power supplies. However, larger driving currents are required since load circuits have increased in size and the parasitic inductance can no longer be ignored.
Parasitic inductance slows down the effective speed of the drivers. Such inductance introduces an unwanted oscillation in the output of the drivers and in driver power supply wiring. This oscillation is caused by an effective LC circuit that exists in the connection between the inherent or parasitic inductances and the capacitance of the load. The output of the driver will not be a valid output level until the oscillations have decayed to a level below that rated output level. To achieve that level, others have simply used smaller output drivers, or delay turning on the output to dampen the oscillations. However, faster output responses require larger drivers to handle more current in less time. Such large currents result in either extended oscillations that slow down the valid output or result in an unstable, oscillating output.